1. Field of the Invention
The present invention relates generally to direct sequence/code division multiple access (DS/CDMA) cellular mobile communication systems, and more specifically to a DS/CDMA receiver which is particularly suitable for use in cell-site stations.
2. Description of the Related Art
Commercial interest in DS/CDMA communication systems has recently risen dramatically due to their potential ability to provide service to more users than is offered by other access techniques. In the DS/CDMA system, each mobile station includes a channel encoder and a multiplier for multiplying the output of the encoder with a distinct spread code. The output of the multiplier is modulated on a carrier for transmission. Signals from mobile stations are code division multiplexed with other signals within the same frequency spectrum, and the signal at the input of each DS/CDMA cell-site receiver is therefore a sum of the signals from the mobile stations. At a cell-site station, a plurality of CDMA receivers are provided, one for each mobile station. Each cell-site receiver multiplies a received bit sequence with a distinct spread code, which may be preassigned or assigned on an as-needed basis, so that signals from undesired mobile stations are "spread" over the frequency spectrum of the system and become noise, while the components of the desired signal are "despread" into a signal having a significant amplitude. The noise resulting from the spreading of undesired signals may be insignificant if the number of mobile stations is small. However, the noise level will have an interfering effect on the desired signal if the number of mobile stations increases.
For interference cancellation, a multiuser DS/CDMA receiver that utilizes the spread codes of all mobile users is described in a paper titled "Near-Far Resistance of Multiuser Detectors in Asynchronous Channels", Ruxandra Lupas et al, IEEE Transactions on Communications, Vol., 38, No. 4, Apr. 1990, pages 496-508. However, it is complex and impractical in certain cases because each mobile user must have the knowledge of all other spread codes. A single user receiver using the spread code of its own receiver for interference cancellation is described in a paper "Equalization for Interference Cancellation in Spread Spectrum Multiple Access Systems", M. Abdulrahman et al, Proceedings of VTC '92, May 1992, pages 71-74. This single user receiver cancels interference by using an equalizer. The output of the equalizer is applied to a decision circuit whose output is coupled to a channel decoder. A matched filter (despreading filter) may be provided between the band-limiting filter and the equalizer and a spread code is applied to the matched filter as tap weight coefficients to extract the desired signal, while the interference that remains at the output of the matched filter is eliminated by the equalizer.
However, since the equalizer's tap weight control is performed at the same time with carrier synchronization, there is an undesired mutual interaction between phase tracking and interference cancellation in a high-speed fading environment, causing a loss of phase synchronization.
On the other hand, the RAKE receiver is well known as an optimum receiver for multipath environment. Since the RAKE receiver attempts to collect the signal energy from all the received signal paths that fall within the span of its delay line and carry the same information, its action is somewhat analogous to an ordinary garden rake, and hence, the name "RAKE receiver" has been coined. However, the prior art RAKE receiver was not designed with interference from other signal sources in mind. An adaptive RAKE receiver is described in a paper titled "BER Performance of Adaptive RAKE Diversity (ARD) in DPSK DS/CDMA Mobile Radio", Akihiro Higashi et al., Proceedings of ISSTA '92, November-December, 1992, pages 75-78. The adaptive RAKE receiver provides diversity combining of all arriving signals by weighting them with tap weight coefficients which are determined with other signal sources in mind so that the desired signal can be extracted efficiently, instead of recognizing all the temporal components of a matched filter as a desired signal and combining them with equal gain. However, the signal detection at every instant is determined exclusively by the use of a matched filter, and the interfering signals are not cancelled during the time prior to the diversity combining.
In summary, any of the prior art DS/CDMA receivers is not satisfactory in carrier phase tracking and interference cancellation performance under high-speed fading and multipath environments.